1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, and specifically relates to the image forming apparatus having two-sided recording function for forming images on both surfaces of a sheet.
2. Description of the Related Art
In order to record on both surfaces of a sheet in an image forming apparatus of an electrophotographic type such as a copying machine and a printer, the conventional apparatus has the structure to be described below.
The conventional apparatus is provided with a two-sided conveying mechanism of a switch back type. The mechanism conveys the sheet to a transfer portion, where, the sheet abuts an image bearing member, after forming a toner image on the image bearing member, and the toner image is electrically transferred to the sheet, thereby transferring the toner image to a first surface of the sheet, and the mechanism conveys the sheet to a fixing unit to fix the image on the first surface. After that, the mechanism reverses the sheet by switch back, and conveys the same to a two-sided conveyance path, such that the toner image is transferred to a second surface of the sheet.
Meanwhile, in a case in which the images are recorded on both surfaces of the sheet, it is required to record transfer images of the first and second surfaces without positional misalignment relative to the sheet.
And there is the two-sided conveying mechanism, in which positional misalignment is corrected in the two-sided conveyance path, through which the sheet is conveyed to be transferred the toner image to the second surface thereof after fixation of the toner image to the first surface thereof. This is to correct the skew feeding when the sheet is conveyed with declination relative to the conveying direction, and to prevent positional misalignment in a sheet width direction, which is orthogonal to the sheet conveying direction. For example, a mechanism, in which an end portion of one side of the sheet is struck to a abutment member, being a horizontal registration reference by a sheet shifting unit, as a position aligning unit in the sheet width direction, is suggested as disclosed in Japanese Patent Application Laid-Open No. 2001-175037.
However, in the above-described two-sided conveying mechanism of the switch back type, front and rear of the sheet relative to the conveying direction switches. Therefore, the tip end of the sheet when entering into the transfer portion when transferring to the first surface and the tip end of the sheet when entering into the transfer portion when transferring to the second surface are tip ends of different sides. So, if a length of the sheet in the conveying direction is different, misalignment in the conveying direction might be generated between the image position of the first surface and that of the second surface, when transferring the toner image on the transfer belt at a predetermined timing, by detecting the tip end of the sheet. And, if there is a positional misalignment between the image of the first surface and that of the second surface, there is a problem that a length of a margin on an upper portion of the sheet, on which the image is formed, is different between the first and second surfaces, so that it is not well-looking.
Therefore, there is the two-sided conveying mechanism disclosed in Japanese Patent Application Laid-Open No. 2002-020000. This is the two-sided conveying mechanism of a twist reversal type, in which the sheet is reversed by 180° in the sheet width direction so as to reverse only two surfaces of the sheet, and front/rear of the sheet relative to the conveying direction is not reversed, in order to improve a positional accuracy of the tip end of the image of the first surface and the tip end of the image of the second surface. In this mechanism, since the front and rear of the sheet is not switched by reversing the sheet, the positional alignment of the tip end of an image region on an upper portion of the first and second surfaces of the sheet may be performed with high accuracy.
However, the above-described conventional example has a problem as follows.
In the image forming apparatus having the two-sided conveying mechanism of the twist reversal type, the mechanism reverses the sheet by 180° in the sheet width direction while conveying the same, so that a large space is required in the sheet conveying direction and in the direction orthogonal to the sheet conveying direction. Especially, the larger the size of the sheet capable of image forming is, the larger the required space is. Therefore, a width and a height of the image forming apparatus main body become larger, so that the image forming apparatus itself becomes larger.